Design a subscription model with service credentials

[s-tikhomirov]

Paying for each request with a separate transaction, albeit simple, may be unacceptable due to latency (#54). We may consider a subscription model instead, where a client pays for multiple future requests, and then provides a proof (a service credential) that the subscription is still valid.

A service credential scheme theoretically allows for more privacy, as payment is separated from getting the service - see a separate issue #60 .